exporting multiple animated objects to gltf
A newer manual says:
&quot;An NLA Strip animation consists of multiple actions on multiple objects that should play together. To create an NLA strip animation with the name “My Animation”, push the action that should play each object onto an NLA track for that object with the name “My Animation”.
NLA Strip animations will be exported if the Animation ‣ NLA Strips option is selected (on by default). All glTF animations are imported as NLA Strip animations.&quot;
Or see here: https://github.com/KhronosGroup/glTF-Blender-IO/issues/219#issuecomment-453621051
The second part of that post:  &quot;If you have two NLA tracks with the same names on different objects, you should get a single merged animation out that affects both.&quot;
So you have to create NLA tracks and give them the same name. It worked with my animation. good luck with yours!